1. Field of the invention
The present invention refers to a method and system for detecting the geometry and dimensions of underground fractures, in particular capable of determining the length and height of such fractures.
2. Description of the Background
In the field of the extraction of hydrocarbons, such as oil and/or natural gas, the production of a well can be improved by causing the hydraulic fracturing of the underground formation of hydrocarbons through the injection of a suitable pressurised liquid into the well itself. A filling material called proppant is then inserted into the produced fracture that prevents it from closing when the hydraulic pressure applied stops.
The volume filled with proppant constitutes the hydraulically conductive part of the induced fracture. Therefore, knowing, in real time or a posteriori, the geometry, in particular height and length, and relative other dimensions of such a volume or proppant pack is considered a key element in determining the quality of the treatment with the relative evaluations on the possible further actions to undertake and on the productivity of the well. Other fields of application in which methods for estimating or detecting the geometry of underground fractures are currently used are, for example diagnostics of other techniques for stimulating the production of hydrocarbons, like matrix stimulation in calcareous formations known as wormholes or acid frac type stimulation.
More generally, such methods can be extended to the detection of fractures made for different purposes and fields, like for example searching for water.
Currently various methods for estimating or detecting the geometry of underground fractures are known that do, nevertheless, have limitations in applicability and in reliability and accuracy of the results.
For example, indirect estimation methods are known the correctness of which is linked to the reliability of the estimation models used and to the accuracy of the data used to supply the models.
Preferably, such models are based on the net pressure values used during the fracturing of the formation. In this case, the reliability of the results is closely linked to knowing the net pressure used for fracturing, which is not always available in accurate terms. Alternatively, it is possible to estimate the geometry of an underground fracture based on its contribution to the productivity of the well.
In order to obtain reliable data, it is however necessary to have an ample historical set of data on the productivity prior to the creation of the fracture, for example relating to a time period of many months or years, in order to be able to accurately determine the increased productivity after its creation.
Moreover, the portion of fracture that actually contributes to productivity may not correspond in terms of dimension and geometry to the entire fracture generated during hydraulic fracturing. Consequently, the length of the fracture is generally underestimated.
In addition to the estimation methods based on models, methods for detecting the geometry of an underground fracture are known that can be divided into many categories.
According to a first type it is foreseen to carry out remote measurements through special instruments, like for example tiltmeters suitable for detecting the inclination of a surface generated by the fracture or else receivers capable of detecting the microseismic events connected to the creation of the fracture.
The detection instruments are placed on the surface or else underground in monitoring wells, specially made or already existing, next to the production well in which the fracture is generated.
From the mapping of the measurements deriving from such instruments it is possible to identify the deformation of the subsoil caused by the generation of the fracture and consequently the geometry of the fracture itself.
Such methods, whilst being amongst the most advanced, have limitations of use and are difficult and expensive to apply due to the need to use specific instruments, the laboriousness of their arrangement and possibly the need to make a special monitoring well.
A second type of method for detecting the geometry of an underground fracture count on that the measurement be carried out directly from the production well in which the fracture is generated.
A first of such methods uses radioactive isotopes that are injected together with the filling material and act as markers of the geometry of the proppant pack present in the fracture created.
However, as well as the risks connected to the use of radioactive material, the detection can only be carried out for a relatively short time period equal to the decay time of the isotopes. Moreover, marking with radioactive isotopes is only able to provide an accurate measurement of the geometry of the portions of proppant pack immediately near to the well.
Therefore, this method, just like others based on measuring the temperature or the flow of fluids carried out in the area of the well near to the fracture, is unable to provide a measurement of the length of the proppant pack and therefore of the fracture.
A second method for detecting the geometry of an underground fracture carried out directly from the production well also counts on using a special filling material.
In this case the filling material is enriched with particles able to improve the transmission or reflection of an electromagnetic wave.
Detection occurs through special electromagnetic wave transmitters and respective receivers placed in the well at the opening of the fracture.
The geometry of the fracture is detected through the analysis of the echo signal detected by the receivers.
Although this method is able to offer a measurement of the length of the fracture, it does have various drawbacks.
Indeed, the use of a filling material with special added passive or active particles, as well as entailing substantial time and cost to obtain a material with optimal wave transmission and reflection characteristics, can reduce the production capacity of a fracture.
Indeed, the added particles, in particular those able to improve the reflection of the electromagnetic wave on the walls of the fracture, become deposited on such walls forming an inner coating that can reduce the permeability thereof to the formation fluid.
Currently it is therefore only possible to obtain measurements of the length of the fracture through measurements carried out directly from the production well by potentially giving up a part of the desired increase in production due to the formation of an underground fracture.
The purpose of the present invention is to avoid the aforementioned drawbacks and in particular to make a method for detecting the geometry of underground fractures that is able to offer a reliable measurement of the length and height of the fracture whilst not reducing the production capacity thereof.
Another purpose of the present invention is to provide a method for detecting the geometry of underground fractures that can be applied directly from the production well without using additional monitoring wells.
A further purpose of the present invention is to make a method for detecting the geometry of underground fractures that can be applied with the filling materials generally used for the formation of an underground fracture, therefore without the need for special additives or the introduction of passive or active target particles, to be detected as indicators of the geometry of the fracture.
Another purpose of the present invention is to make a method for detecting the geometry of underground fractures that is able to provide reliable measurements even in the case of a high length of the fracture, i.e. at a substantial distance from the accessible section of the well.
The last but not least purpose of the present invention is to devise a system for implementing the method for detecting the geometry of underground fractures.